1. Field of the Invention
The present invention relates to rotational position sensors, and more particularly to sensors of the type in which varying portions of a light beam emitted by a light emitting diode or other source of illumination are directed onto one or more photoelectric cells to represent angular position of a rotatable element.
2. History of the Prior Art
Rotational position sensors have found use in a wide range of different applications. For example, in the case of magnetic tape decks it is not uncommon to provide a pair of arms for sensing tape tension adjacent supply and takeup reels on opposite sides of a tape driving mechanism such as a capstan. The supply and takeup reels are commonly driven by motors coupled to servo systems which are responsive among other things to the angular positions of the tension arms. In such situations it is desirable to be able to equip each of the tension arms with a rotational position sensor capable of producing an output which varies linearly with the angular position of the tension arm and which is at the same time of low cost and relatively maintenance free construction, has a minimum of moving parts, and has low inertia so as not to substantially interfere with the operation of the tension arms.
Rotational position sensors find similar application when used with the tension arms in certain cameras such as those of the microfilm type used in data processing operations. In such instances it is desirable to drive the reel motors for the film in accordance with the behavior of the tension arms to prevent stretching of the film. Stretching exposes and ruins the film. By the same token it is necessary that the film be sealed from exposure to any light sources used in conjunction with rotational position sensors for the tension arms, or alternatively that light sources be used which do not affect the film such as those within the infrared range.
A number of prior art rotational position sensors having some of the desirable properties noted above have been proposed. One such type of sensor positions a light emitting diode or other source of infrared radiation adjacent the rotatable portion of the tension arm or other element so as to direct a beam therefrom onto a mirror mounted for rotation with the tension arm. As the tension arm rotates the mirror reflects greater or lesser portions of the beam onto one or more photocells disposed adjacent the tension arm. Sensors of this type provide an output signal which varies generally linearly with the angular position of the tension arm. For film applications the infrared light sources do not expose the film thereby obviating the need for expensive or cumbersome light shielding. However sensors of this type suffer from a number of disadvantages which may seriously limit their usefulness for many applications. For one thing such arrangements are needlessly complex because of the problem of properly aligning the various components including the mirror so as to achieve the desired beam reflection. As a result frequent maintenance may be required. The presence of the mirror adds considerable inertia which may be a highly undesirable factor with some tension arms. A still further problem is the space often consumed by sensors of this type. Since light intensity is knonw to vary with the square of distance it is often necessary to locate the light emitting diode or other source and the photocells a substantial distance from the mirror so as to provide little change in the total distance of the light path from the diode through the mirror to the photocells throughout the required angular range of operation.
Ideally the light emitting diode or other source of illumination should be mounted directly on the rotatable element such as the tension arm so as to project a beam directly onto one or more photocells. Such arrangement has the advantages of simplicity, compactness, low cost, low maintenance and low inertia. However arrangements of this type have been considered impractical or impossible, among other reasons because of the problem of varying light intensity and the consequent problem of producing a linear output response.